Individuals of many species undergo seasonal changes in brain and behavior that evolved to optimize reproduction and survival. Photoperiod (day length) is used to determine time of year. Melatonin, secreted from the pineal gland in proportion to night-length, transduces photoperiodic information to evoke a suite of physiological, morphological, and behavioral changes that promote winter-survival. Hippocampal volume decreases in short days, a morphological change that may underlie seasonal deficits in spatial learning and memory. To evaluate the hypothesis that photoperiod mediates behavioral changes via melatonin, we will compare behaviors of deer mice exposed to long or short days or treated with melatonin. The specific aims of the proposed research are: (1) To determine whether short day-induced deficits in spatial learning and memory are mediated by melatonin. (2) To relate these photoperiod-induced deficits to structural changes in the hippocampus. (3) To determine whether photoperiod also affects non-spatial learning and memory tasks. This research will provide insights into the mechanisms of seasonal plasticity of brain and behavior